In recent years, an artificial photosynthesis technology of electrochemically converting sunlight into a chemical substance in imitation of photosynthesis of plants is under development from viewpoints of energy problem and environmental problem. The case of converting the sunlight into the chemical substance and storing it in a cylinder or tank has advantages that the storage cost of energy can be reduced and the storage loss is small as compared with the case of converting the sunlight into electricity and storing it in storage batteries.
As a photoelectrochemical reaction device that electrochemically converts sunlight to a chemical substance, there has been known, for example, a two-electrode type device that includes an electrode having a reduction catalyst for reducing carbon dioxide (CO2) and an electrode having an oxidation catalyst for oxidizing water (H2O), and in which these electrodes are immersed in water in which CO2 is dissolved. These electrodes are electrically connected to each other via an electric wire or the like. In the electrode having the oxidation catalyst, H2O is oxidized by light energy, whereby oxygen (½O2) is obtained and a potential is obtained. In the electrode having the reduction catalyst, by obtaining the potential from the electrode in which the oxidation reaction is caused, CO2 is reduced and formic acid (HCOOH) or the like is produced.
A photoelectrochemical reaction device using a stack (silicon solar cell or the like) which includes a pair of electrodes and a photovoltaic layer sandwiched between the pair of electrodes is also under investigation. In the electrode on a light irradiated side, water (2H2O) is oxidized by light energy, whereby oxygen (O2) and hydrogen ions (4H+) are obtained. In the opposite electrode, by using the hydrogen ions (4H+) generated in the electrode on the light irradiated side and a potential (e−) generated in the photovoltaic layer, hydrogen (2H2) or the like is obtained as a chemical substance.
As described above, production of a chemical substance high in utilization such as oxygen, hydrogen, and a carbon compound such as formic acid and so on from the sunlight is performed in the photoelectrochemical reaction device. However, considering energy problem and environmental problem, it is further required to increase the kinds of products, improve their qualities, and improve the conversion efficiency and so on.